Thermotropic liquid crystal polymer has liquid crystal property at specific temperature range. Typical example of thermotropic liquid crystal polymer is aromatic polyester. It is produced by using terephthalic acid and hydroquinone or p-hydroxy benzoic acid and is represented by the following formula: ##STR4##
Examples of the methods for preparing liquid crystal polyesters generally include the condensation of aromatic diacid with aromatic diacetate and the condensation of aromatic diacid chloride with aromatic diol. The first method has demerits in that it has poor reactivity so that it should be carried out at very high temperature (&gt;400.degree. C.) under reduced pressure and the reactants should have good heat stability. The second method has good reactivity. However, it has disadvantages in that an excess of tosyl chloride should be used in order to prepare aromatic acid chloride, which makes the process complicated and costly. In addition, the melting points of the polymers prepared by the above methods are higher than decomposition temperature and the polymers are insoluble in solvent so that a great difficulties resides in processing those polymers.
Examples of methods for preparing liquid crystal polymers which can be melt-processed by lowering melting point of polymers are as follows:
Firstly, a method wherein bulky substituent is added into aromatic ring to make a polymer having wide intermolecular space is disclosed in German Patent 2,751,653. ##STR5##
Secondly, a method wherein aromatic compounds having different size are added into main chains to reduce interaction between main chains is disclosed in U.S. Pat. No. 4,067,850. ##STR6##
Thirdly, a method wherein flexible aliphatic compound is added into main chain is disclosed in U.S. Pat. No. 3,778,410. ##STR7##
Fourthly, a method wherein non-linear compound is added into main chain to make it bent. ##STR8##
The above polymers have liquid crystal property and good physical property. However, they have no economical merit since they are made from very expensive intermediates.
Therefore, a liquid crystal polyester has been needed which can be prepared in a simple manner and low expense, has good processability and act additionally as functional group by means of double bond.